1. Field of Invention
This invention relates to a color changeable pixel. More particularly, this invention relates to the color changeable pixel of an optical interference display plate.
2. Description of Related Art
Due to the properties of being light and small in size, a display plate is favorable in the market of the portable display and displays with space limits. To date, in addition to Liquid Crystal Display (LCD), Organic Electro-Luminescent Display (OLED) and Plasma Display Panel (PDP), a module of the optical interference display has been investigated.
Referring to U.S. Pat. No. 5,835,255, an array of modulator of the visible light which can be used in a display plate has been revealed. FIG. 1 shows a cross-section view of a prior art modulator. Every modulator 100 comprises two walls, 102 and 104. These two walls are supported by post 106, and a cavity 108 is subsequently formed. The distance between these two walls, that is, the length of cavity 108, is D. One of the wall 102 and wall 104 with an absorption factor is a semi-transparent layer which absorbs visible light partially. Another is a light reflective layer which is deformable when the voltage is applied. When the incident light goes through wall 102 or 104 and arrives at cavity 108, only the visible light with the wavelength corresponding to the formula 1.1 is able to be output, that is,2D=Nλ  (1.1),                wherein N is a natural number.        
When the length of cavity 108, D, equals half of the wavelength times any natural number, a constructive interference is produced and a sharp light wave is emitted. At the mean time, if the observer follows the direction of the incident light, a reflective light with wavelength λ 1 can be observed. Therefore, modulator 100 is “opened”.
FIG. 2 shows a cross-section view of a modulator after a voltage is applied. As shown in FIG. 2, because of the voltage wall 104 is deformed and falls down towards wall 102. The distance between wall 102 and 104, that is, the length of cavity 108 is not exactly zero. It is d, and d can be zero. If we used d instead of D in formula 1.1, only the visible light with a wavelength fulfilling formula 1.1, which is λ 2, is able to produce a constructive interference and goes through. Due to the high absorption rate of wall 102 for light with wavelength λ 2, all the incident visible light would be filtered, therefore the observer who follows the direction of the incident light is not able to observe any reflected visible light. The modulator is now “closed”.
An array of modulators comprising modulator 100 is sufficient for a single colored display plate, but not for a color planar display. A method known to the art is to manufacture a pixel which comprises three modulators with different lengths of the cavities. FIG. 3 and FIG. 4 are cross-section views for the color planar displays comprising modulator known to the arts. FIG. 3 shows a cross-section view for a prior art multi-layered color planar display. Multi-layered color planar display 200 comprises three layers, modulators 202, 204 and 206. An incident light 208 is reflected by modulators 202, 204 and 206. The wavelengths of the reflected light are different, for example, they can be red light, green light and blue light. The reasons to have reflected light with three different wavelengths is that the length of the cavities of modulators 202, 204 and 206 are different, and also different reflective mirrors are used. One of the disadvantages of a multi-layered color planar display is its poor resolution. Also, as shown in FIG. 3, the blue light is less bright than the red light.
FIG. 4 shows a cross-section view for a prior at matrix color planar display. Three modulators, modulators 302, 304 and 306 are formed on a substrate 300. An incident light 308 is reflected by modulators 302, 304 and 306. The wavelengths of the reflected light are different, for example, they are red light, green light and blue light. The reason to have reflected light with three different wavelengths is that the lengths of the cavities of modulators 302, 304 and 306 are different. It is not required to use different reflective mirrors. The resolution is good, and the brightness of every color light is similar. However, modulators with three different lengths of cavities need to be manufactured separately, for example, the region for forming the modulators 304 and 306 is shielded by photo-resist while the process for forming the modulator 302 is performed. The manufacturing process is complicated and the yield is low. Moreover, the errors introduced during the manufacturing process, for instance, the errors of the lengths of cavities may cause red shift or blue shift. The mistake is uncorrectable and the substrate is wasted.
Therefore, it is important to develop a color optical interference display plate which has high resolution and brightness and is easy to manufacture.